A photonic crystal fiber with As2S3 glass core and tellurite glass micro-cladding was designed. This design is a perfect conbination of both traditional step index fiber and photonic crystal fiber and own the characteristics of high dispersion adjustment flexibility and low confinement lose. Simulation results show that, by carefully optimizing the characteristic structure parameters such as the fiber cladding air holes diameter , air hole pitches and fiber core diameter of this fiber, and while letting the parameters above meet linear relationship with fiber length, the dispersion profile of this fiber will be flattened and decreasing with fiber length in mid-infrared region of 2～4.5 μm. In addition, we optimized the dispersion of this fiber by filling the air holes of micro-cladding with oil which linear index is around 1.6. Fiber with such novel dispersion property is rare and will be of great importance in applications of laser pulse compression and stretching, dispersive wave generation, soliton and mid-infrared super-continuum generation.